Ethyl-tertiary-butyl ether (ETBE) is recognized as a suitable blending cosolvent for hydrous ethanol in gasoline stocks. See U.S. Pat. No. 4,207,076 (June 10, 1980). ETBE is blended into a fuel gasoline at about a 10 to 20 volume percent level, more usually about 9 to 12 percent, in which the fuel comprises about 70 to 84 percent gasoline and 5 to 20 percent of 95 percent ethanol (grain alcohol). ETBE solubilizes grain alcohol in gasoline in all proportions thereby allowing a wide latitude in the precise amount of ethanol which can be blended with the gasoline. In addition the presence of ETBE in the blend considerably increases its octane rating, both motor and research.
Methyl-tertiary-butyl ether (MTBE) is a recognized blending stock for raising the octane number of gasoline. MTBE, blended into gasoline at about a 10 to 20 volume percent level, increases both motor and research octane numbers by several units. The Environmental Protection Agency has sanctioned the use of MTBE in concentrations up to about 7 liquid volume percent in the United States.
ETBE and MTBE are produced by reacting isobutylene with either ethanol or methanol, resulting respectively in the formation of ETBE or MTBE. The reaction normally is conducted in liquid phase with relatively mild conditions. While mixed butylenes streams can be employed, only the tertiary olefin, isobutylene, reacts at the conditions employed. The isobutylene can be obtained from various sources, such as naphtha cracking, catalytic cracking, etc. Refer Hydrocarbon Processing, Dec., 1979, pages 109 and following, and The Oil and Gas Journal, Apr. 9, 1979, pages 149 and following.
The resulting reaction product stream contains the desired MTBE or ETBE, as well as unreacted isobutylene and methanol or ethanol. A problem that persists has been the separation of the unreacted alcohol from the MTBE or MTBE product. At equilibrium conversion, considerable alcohol remains in the reactor effluent, is difficult to remove by simple distillation, and may form troublesome azeotropes with ETBE and MTBE.